As is well known, horizontal axis wind turbines are used extensively in industry. Typical horizontal axis wind turbines comprise a rotor wherein at least two blades are attached extending radially from a hub, a nacelle which supports the rotor on a main axle, which extends in essentially the horizontal direction, and which is connected to the hub, and a turbine holding member such as a tower that is disposed in essentially the vertical direction, and that supports the nacelle so as to be rotatable in the yaw direction. Additionally, horizontal axis wind turbines conventionally have also been provided with yaw driving means that can be controlled to drive the rotation of the nacelle in the yaw direction, and braking means such as yaw brakes for slowing/stopping the yaw rotation and main brakes for slowing/stopping the rotation of the rotor. Horizontal axis wind turbines equipped with means for controlling the blade pitch angle are also used.
Normally the design strength of the turbine is influenced greatly by the load that it will receive when in the standby state during a windstorm. The turbine load during a windstorm must also be set envisioning a concomitant power outage. Additionally, usually the power generation is stopped during a windstorm, so the horizontal shaft wind turbine is idle. In a conventional upwind pitch control device, after feathering has been secured for all blades, the nacelle orientation angle is reversed by approximately 180° by the yaw control and held by a weak yaw brake, to be in standby at the time of the windstorm. (See, for example, non-patent reference 1.) Doing so makes it possible to face the rotor downwind during the windstorm, to thereby reduce the load on the turbine.
Non-patent reference 1: SHIBATA, Masaaki and HAYASHI, Yoshiyuki: “New Concepts for Reducing Design Load,” 25th Wind Power Energy Use Symposium, 20 Nov. 2003, Pages 225 to 227
The typical wind turbine of today, including the conventional technology described above, is a pitch-control wind turbine wherein the pitch angle is controlled. This type of wind turbine typically feathers the pitch angle and allows the rotor to rotate freely to go into windstorm standby. When it comes to yaw control, there are those wind turbines wherein yaw is controlled actively, those wherein it is controlled passively, and those wherein it is not controlled at all (held by a yaw brake/yaw worm gear). In any wind turbine wherein yaw control is possible, if the power required for yaw control is interrupted, or when there is a failure in one of the mechanisms relating to the yaw control, there is the possibility that the storm wind can be received from any direction. As a result, it is necessary for the design to envision a windstorm from all directions. Large wind turbines have a greater propensity towards stall flutter. When one envisions the windstorm coming from all directions, there will be large stalling flutters in specific wind directions relative to the nacelle, producing large loads.
The present invention is the result of contemplation of the problem areas with the conventional technology, described above, and the object thereof is to provide a horizontal axis wind turbine capable of reducing flutter, and by extension, reducing the load on the wind turbine, without controlling the yaw, regardless of the direction of the wind relative to the nacelle.